18 | MOLTEN METAL
w w w. m m c p u b l i c a t i o n s . c o . u k
Beliefs about fused magnesium
chloride fluxes questioned
Chlorine gas has largely been replaced by magnesium chloride fluxes as a means of reducing alkalis and oxides from
aluminium melts. Two widely held beliefs concerning the composition and effects of these fused fluxes have developed in
recent years and this article explains why they should be questioned*.
Over the last fifteen years fused magnesium chloride fluxes have largely replaced chlorine gas as a
means of reducing alkalis and oxides from molten
aluminium. The change, hastened by environmental considerations, began after publication of an
article by Beland(1) in 1995. This described a refining flux based on the binary system magnesium
chloride-potassium chloride, where a magnesium
chloride intermediate was the rate controlling
species active in the removal of alkalis.
Refining fluxes were at first based on the binary
magnesium chloride-potassium chloride system
with two low melting point eutectics at about 55
and 36 mole magnesium chloride. Later a revised
binary diagram was accepted as a more accurate
description of the magnesium chloride system with
a third eutectic occurring at 31 mole. Accordingly,
commercial products became accepted based on
the three eutectics with magnesium chloride contents ranging from the slightly hypo eutectic 25%
by weight up to the hyper eutectic 75% by weight.
Two widely held beliefs, have developed about
fused magnesium chloride fluxes.
MgCl percentage
The first belief is that the performance in terms of
efficiency of sodium removal is directly related to
the percentage of magnesium chloride in the fused
flux. It is well established that in fused magnesium
chloride-potassium chloride fluxes magnesium
chloride is the active component participating in
the reaction:
MgCl2 + 2 Na = 2 NaCl + Mg
KCl on the other hand plays no part because as
KCl is more stable than NaCl, having a higher free
energy of formation and based on this the belief
existed that the performance in terms of efficiency
of sodium removal should be higher the higher the
percentage of magnesium chloride in the product.
In a number of separate series of full scale casthouse trials involving direct comparisons under the
same conditions it was demonstrated that the performance of 60%, 40% and 25% MgCl2 flux compositions all produce equivalent results. Final
confirmation came when David de Young (2) published the results of an investigation carried out at
the Alcoa Technical Centre into the same subject
and showed that varying the % MgCl2 between
10% and 90% in fused magnesium chloride-potassium chloride fluxes had no influence on the rate of
sodium removal as shown in fig.1.
In explanation of this Dietze(3) has proposed that
the concentration of MgCl2 in the molten salt
*Article based on a paper being presented by MQP
at the 2011 Australian Asian Pacific Aluminium
Casthouse Technology Conference, Melbourne.
1. Effect of % MgCl2 on sodium removal efficiency (after De Young)
droplet has little influence on the reaction kinetics
because the rate of salt addition applied is ten times
that needed to satisfy the requirement for stoichometric reaction. Of far greater importance are the
kinetic factors and therefore in order to achieve an
adequate distribution of a relatively small amount
of flux in the aluminium melt it is necessary to add
considerably more than the required stoichometric
amount. Thus the concentration of MgCl2 in the
individual salt droplets, providing that it is always
greater than that required to satisfy the reaction,
has little influence on the reaction kinetic.
The conclusion from such comprehensive trials
and examination of reaction kinetics is that the performance in terms of efficiency of sodium removal
is not related to the percentage of magnesium chloride in the fused flux.
2. Ternary system MgCl2 – KCl – NaCl showing new salt flux composition
Aluminium Times September 2011
Sodium chloride content
The second widely held belief is that the amount of
sodium chloride permitted in fused salt products
for sodium removal from aluminium melts should
be restricted below 1 per cent. When fused salt
fluxes were first introduced potassium chloride was
a suitable low cost material to combine with the
more expensive MgCl2 component to provide a low
melting point eutectic. However, this dramatically
changed with the price of KCl increasing by more
than 350% up to a peak of nearly US$ 900 per
tonne in July 2009 due to global demand for potash
fertiliser for the production of grain crops and biofuels. It seems inevitable that KCl prices will rise
again resulting in cost increases for both producers
and users of fused salts. This has led to a reassessment on the possible role of NaCl in fused salts
20 | MOLTEN METAL
where it would appear to be an ideal substitute or
partial substitute for KCl.
The reaction, MgCl2 + 2 Na = 2NaCl + Mg,
can be considered to move strongly to the right
which means that sodium removal by MgCl2 is
practically effective. Ellingham diagrams confirm
the high stability of NaCl with respect to MgCl2. It
can be anticipated that in practice there would be
no increase in sodium in the aluminium if quantities of NaCl were introduced into the flux composition. The above hypothesis was tested by
thermodynamic modeling of the reaction. A ternary
product composition was selected, with an appropriate addition of NaCl to an existing proven formulation, by reference to the ternary diagram in fig 2.
A thermodynamic modelling study was undertaken
with IME Aachen to investigate the effect of a sodium chloride content from 1 to 25 % in a magnesium–potassium chloride fused flux, on the
removal of up to 30 ppm of sodium from an aluminium melt. Modelling showed that no sodium
remained in the melt after reaction and therefore it
can be concluded on the basis of the thermodynamic calculations that up to 25% NaCl can be
substituted for KCl in Refinal without any effect on
the residual Na content after treatment. Full scale
casthouse trials were then carried out at a major
casthouse in Europe by comparing Refinal, containing 25% sodium chloride, with the standard
practice using a conventional Refinal 35% MgCl2,
65% KCl fused flux. Over a total of 87 furnace
preparations an average Na level of 6.5 ppm at first
time of batching was achieved with the NaCl containing flux which was equivalent to the results
achieved with their standard practice. The conclusion from thermodynamic modelling and casthouse
trials is that sodium chloride can be added to fused
magnesium chloride-potassium chloride fluxes
without affecting sodium removal from aluminium
melts.
Sensible beliefs
It is not surprising that it was believed that the performance of a flux might be proportional to MgCl2
content. Nor is it surprising that it was believed
that adding NaCl to a flux might lead to Na pick
up, when the objective is to remove Na. Nonetheless impartial scientific investigation has confirmed
otherwise in both cases. In the case of the effect of
MgCl2 the reaction kinetics are of overriding
importance and not the proportion of MgCl2 in the
product. In the case of the effect of NaCl addition,
the thermodynamics confirm that irrespective of
kinetic considerations NaCl cannot be reduced
again to Na in the reaction system involved.
Bibliography
G.Beland, C. Dupuis and J.-P. Martin, “Improving fluxing of Aluminium Alloys” Light Metals,
1995, pp1189-1195.
(2) D.H.DeYoung, “Salt Fluxes for Alkali and Alkaline Earth removal from Molten Aluminium”, 7th
Australian Asian Pacific Conference Aluminium
Casthouse Technology, 2003.
(3) Private communication with Dr. A. Dietze & TU
Clausthal
(1)
Reader Reply No.48
Author: John Courtenay and Michael Bryant,
both of MQP Ltd, UK
w w w. m m c p u b l i c a t i o n s . c o . u k
Thermserve billet heater for
Vimetco in Slatina
Thermserve Limited, based at Telford in the United Kingdom have recently
moved to larger premises which allows the company to not only design
equipment and machinery but to also engineer and build. The company recently
was awarded the Queen’s Award for Enterprise
Vimetco Extrusion SRL, with their headquarters in
Slatina, in Romania, is a subsidiary of VimetcoALRO S.A. which is one of the largest smelters in
Europe. The smelter has been operating since 2006
and Vimetco came to Thermserve for a 2500Kg/hr
178mm billet heating furnace and hot log saw with
specific performance requirements concerning
throughput and fuel consumption. The equipment
is currently undergoing Factory Acceptance Tests in
the Thermserve fabrication shop, in the United
Kingdom.
Thermserve log and billet ovens are
manufactured in their own, well equipped,
fabrication facility. For ease of construction,
maintenance and to accommodate potential
extensions to the service requirements Thermserve
use modular design techniques. For this reason, the
sectional mild steel casing will have readily
removable top sections to access the pre cast
refractory blocks and roller conveyor system which
transport the logs through the furnace.
These steel and precast refractory sections are
constructed with all the necessary apertures for the
various inserted services and instrumentation. The
insulation materials including the precast refractory
blocks are designed to give thermal efficiency
through the full range of the oven capabilities.
The combustion equipment consists of a range of
burners and is divided into independently
controlled zones, which are coordinated with the
overall modular design of the oven. Each zone is
equipped with all the required safety systems,
including pilot burners and flame failure devices. A
recuperative section provides recycling of the hot
gasses ensuring efficient combustion and heat
transfer. Close to the discharge end of the
recuperative zone and within each fired zone
Thermserve provide pneumatically operated
thermocouples to contact the surface of the log. The
operation of the thermocouples is synchronised
with the passage of the log through the furnace and
sequenced by the PLC control system as with the
main body of the oven the combustion system is
design with ease of inspection and routine
maintenance in mind.
Essential to the efficient operation of the log/billet
heater is the functional furnace pressure control to
maintain a balanced pressure within the heating
chamber and eliminate cold air entry, therefore an
electronic transmitter is connected to a sensing
point near the discharge end. The transmitter
actuates an exhaust damper through an electrical
modulating motor, so that the damper closes when
the burners are turned down, and opens as the
burners move to maximum firing rate. Pressure
control is assisted by the provision of an actuated,
insulated door at the discharge end of the furnace
where a photocell is located to control movement
for the pusher gear and to stop each log in a fixed
Aluminium Times September 2011
Combustion system controls.
position. Thus no adjustment of controls is
necessary when changing log length.
All of the combustion and control features of a
Thermserve log/billet oven are designed to provide
the required level of information to the control and
Instrumentation centre in order to provide
intelligent temperature control. The control centre
is designed, built and thoroughly tested in
Thermserve’s in house panel shop to optimise the
temperature and to manage the in feed system with
the following features:• Automatic temperature control for each of the
fired zones of the furnace.
• Control of all log movement sequencing from
storage through the heater.
• Integration of furnace and handling equipment.
• An audible alarm that will sound in the event of
abnormal temperature condition in any zone.
• Full integration of a Thermserve Hot Saw
In this case, Allen Bradley PLC equipment, is
specified but similar equipment by other
manufacturers can be utilised if requested.
The Thermserve main panel also houses the
following:• Isolator, giving mechanical and electrical
interlocks with the cubicle door.
• Direct on-line motor starters, overloads, fuses,
etc.
• Flame failure relay and ignition units for the
fired zones.
* HMI – Push buttons, labels, indicator lights,
etc., for control supply and emergency stops.
• The panel is fully wired to terminal strips and
shop tested before despatch.
The equipment for this project has been designed
and manufactured to relevant British Standards (or
European Standards EN/ISO - as appropriate)
To facilitate an efficient installation the log heater
is currently fully assembled in the Thermserve
works and will be dismantled only to the extent
necessary for efficient and safe transportation to site.
Reader Reply No.49